Also you have to consider that some people do not know how to upgrade their system, or indeed what good prices are/whats actually good product!
Then there is also the ADDED danger of issues with the computer....if something goes wrong you either have to be able to troubleshoot yourself, or alternatively get it fixed by someone else (costs alot). You also have to buy everything seperately, which I noticed can be a real pain if you start afresh - XP, Office - and anything else you need.
Simply put those people do not want to have any hassles. They want a computer that will work well, and thats it. If there is a problem, get it picked up and fixed (if under warrenty). Its pretty much a comfort factor - less hassle. I myself prefer to build them, but then again, I don't know
enough to be really competant when it comes to troubleshooting issues. I have had my fair share of problems - including kaput motherboards (brand new - got a refund), 512mb of memory dying (it was 2nd hand), un-known graphics card issues (Raddie 9800 pro - turned out it didn't have enough pooooower on the '12v rail' as they say for my system specs, so it kept freezing the system up needing vpu recover), HDD failure.
Now some of those were my own fault - 2nd hand junk (They did last 1.5 years though...the memory, the hdd) and some was just out of my control (Mobo!), and some was just lack of understanding (Vid Card) - but if you knew less than me, you would have been back and forth to PC Repair places about 10 times
. That would probabily have been the cost of a new comp